Lightness and Brightness and Other Confusions

curve is the foundation of the photometric technology, which is today the
accepted method for specification of light sources and lighting designs. 62
Thus, a photometric definition of light refers to electromagnetic energy
weighed against a theoretical model about sensitivity of the human visual
system to radiation within the so-called visible spectrum. The V( ) curve is
not an absolute truth, but a scientific theory which has been revised a few
times and which has also been fundamentally questioned (Liljefors 2010). So
far, though, the gathered expertise within the CIE have agreed that it is the
best available tool to quantify the light that we see through measurements of
physical radiation. 63
1.0
Spectral efficiency
0.8
0.6
0.4
0.2
0
400 450 500 550 600 650 700
Wavelenght (nm)
Figure 13. The V-lambda curve, V( )
Photometric concepts and units are all based on the V( ) curve and denote
different aspects of photometric light. Luminous flux (measured in lumen)
and luminous intensity (measured in cd = candela) are used for the light
emitted from the light source, illuminance (lux) for the illumination of a
surface and luminance (cd/m 2 ) for the light reflected or radiated to our eye
from a surface. Photometric technology includes several instruments like the
spectrophotometer and the lux meter, all of them measuring radiation and
weighing it against the V( ) curve.
62
The V( ) curve was developed for photopic vision, that is: human vision under full
light conditions defined by a luminance of 3,4 cd/m 2 minimum. For lower light conditions
there are other similar curves that are not further discussed here.
63 Note that the units based on the V( ) curve are not, and do not claim to be, applicable
for measuring “light” that is not received by the human visual sense, e.g. the visual or
otherwise light sensitive senses of animals or the photobiological processes in humans,
animals and plants.
59